10 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



concept of metabolic "pathways'' may be more literal than had 

 been supposed. 



More or less permanent, heavy-duty organelles, such as mito- 

 chondria, nerve myelin, chloroplasts, and the outer segments of 

 vertebrate photoreceptors, are fairly easily comprehended in these 

 terms. For the more evanescent membranous structures of the 

 cell, the picture is not so clear. Detection of the three-ply form 

 requires a high degree of fidelity in preservation and micrography, 

 as well as a lucky angle of sectioning. Viewing the static picture 

 of fixed structures provided by the electron microscope, one can 

 too easily forget the visible activity of the living cell. Here the 

 protozoologist's necessary preoccupation with his organisms as 

 living entities may serve him in good stead. For example, the 

 almost explosive formation of pseudopodia by many ameboid 

 cells indicates a virtually instantaneous production or extreme 

 stretching of surface membranes. Extensive but, up to a point, 

 reversible vacuolization of the cytoplasm in many protozoa when 

 subjected to unfavorable conditions on a microscope slide is a 

 commonly observed phenomenon; the cytoplasm- vacuole inter- 

 face here appears as a membrane in electron micrographs, and 

 certainly represents an area vastly increased over what was present 

 before. And the incessant streaming of organelles within the 

 protoplasm of active cells must preclude the existence in these 

 regions of an architecturally fixed network of membrane-bound 

 spaces. In other words, many of the membranous structures we 

 see in electron micrographs must be transient in form and position 

 if not in construction. One cannot imagine how a cell, and 

 particularly a free-living one, could survive if the production, 

 renewal, and elimination of membranes were not to be accom- 

 plished with great facility and with materials readily at hand. 



Much of this chapter is devoted to a consideration of mem- 

 branous organelles as they appear in protozoa. First, however, 

 we must consider the ground substance (which must contain, 

 among other things, membrane precursors). The cytoplasmic 

 matrix, following the rigors of preparation for electron micro- 

 scopy, appears to consist of very fine filaments and/or granules 

 dispersed in a low-density continuum; these probably include 

 both structural elements and precipitates of protoplasmic com- 

 ponents left more or less in situ as the protoplasmic water and 



